Devices for holding a workpiece to be machined in a specific position in relation to a machine-tool on which they may be fixed usually comprise a fixed jaw having two orthogonal reference surfaces, a jaw movable perpendicularly to one of these surfaces and a clamping screw, also perpendicular to this surface, for clamping the machined workpiece between the fixed jaw and the movable jaw; at present they correspond but very imperfectly to the desired aim; in fact, when they are subjected to a substantial clamping force, bending occurs in the body of the fixed jaw on which the movable jaw slides and this bending causes geometrical faults in the position of the jaws; furthermore, if they are subjected to a repetition of stresses permanent deformations occur which make these same shortcomings permanent.
To prevent this it has been proposed to reinforce and support such a structure by means of a complementary structure intended to receive the reactions of the stresses exerted on the fixed and movable jaws, comprising for this purpose a front part, against which the fixed jaw bears and a rear part having therethrough a tapped orifice through which the threaded portion of the clamping screw of the movable jaw passes and in which it is engaged.
In all the devices known up to present, this complementary structure is formed by an elongated part having its two ends raised, one of which serves as support for the fixed jaw, and the other of which has therethrough a tapped orifice in which the clamping screw of the movable jaw is in meshing engagement.
Now, when such a structure is subjected to stresses caused by energetic clamping of the clamping screw, the elongated part bends which adversely affects the efficiency of the clamping; furthermore, this elongated part is large in size and it requires accordingly a large cut-out in the body of the fixed jaw for housing it therein, which adversely affects the rigidity of this body.